Doped Semiconductor Nanocrystal Layers And Preparation Thereof

ABSTRACT

The present invention relates to a doped semiconductor nanocrystal layer comprising (a) a group IV oxide layer which is free of ion implantation damage, (b) from 30 to 50 atomic percent of a semiconductor nanocrystal distributed in the group IV oxide layer, and (c) from 0.5 to 15 atomic percent of one or more rare earth element, the one or more rare earth element being (i) dispersed on the surface of the semiconductor nanocrystal and (ii) distributed substantially equally through the thickness of the group IV oxide layer. The present invention also relates to a semiconductor structure comprising the above semiconductor nanocrystal layer and to processes for preparing the semiconductor nanocrystal layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 10/761,409, filed on Jan. 22, 2004, entitled “DOPEDSEMICONDUCTOR NANOCRYSTAL LAYERS”, assigned to the assignee of thepresent application, and claims the benefit of U.S. Provisional Patentapplication Ser. No. 60/441,413, filed Jan. 22, 2003 entitled“PREPARATION OF TYPE IV SEMICONCUDTOR NANOCRYSTALS DOPED WITH RARE-EARTHIONS AND PRODUCT THEREOF”, the contents of which are hereby incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates to semiconductor nanocrystal layers dopedwith rare earth elements, to semiconductor structures comprising thesesemiconductor nanocrystal layers, and to processes for preparing thesemiconductor nanocrystal layers doped with rare earth elements.

BACKGROUND OF THE INVENTION

Silicon has been a dominant semiconductor material in the electronicsindustry, but it does have a disadvantage in that it has poor opticalactivity due to an indirect band gap.

This poor optical activity has all but excluded silicon from the fieldof optoelectronics. In the past two decades there have been highlymotivated attempts to develop a silicon-based light source that wouldallow one to have combined an integrated digital information processingand an optical communications capability into a single silicon-basedintegrated structure. For a silicon-based light source (silicon LightEmitting Diode (LED)) to be of any practical use, it should (1) emit ata technologically important wavelength, (2) achieve its functionalityunder practical conditions (e.g. temperature and pump power), and (3)offer competitive advantage over existing technologies.

One material that has gathered much international attention is erbium(Er) doped silicon (Si). The light emission from Er-doped Si occurs atthe technological important 1.5 micron (gm) wavelength. Trivalent erbiumin a proper host can have a fluorescence of 1540 nm due to the⁴¹¹3/^(2−>) ⁴¹15/2 intra-4f transition. This 1540 nm fluorescence occursat the minimum absorption window of the silica-base telecommunicationfibre optics field. There is great interest in Er doping of silicon asit holds the promise of silicon based optoelectronics from the marriageof the vast infrastructure and proven information processing capabilityof silicon integrated circuits with the optoelectronics industry.

Theoretical and experimental results also suggest that Er in Si isAuger-excited via carriers, generated either electrically or optically,that are trapped at the Er-related defect sites and then recombine, andthat this process can be very efficient due to strong carrier-Erinteractions. However, if this strong carrier-Er interaction isattempted in Er-doped bulk Si, the efficiency of the Er³⁺ luminescenceis reduced at practical temperature and pump powers.

Recently, it has been demonstrated that using silicon-rich silicon oxide(SRSO), which consists of Si nanocrystals embedded in a SiO₂ (glass)matrix, reduces many of the problems associated with bulk Si and canhave efficient room temperature Er³⁺ luminescence. The Si nanocrystalsact as classical sensitizer atoms that absorb incident photons and thentransfer the energy to the Er ³⁺ ion, which then fluoresce at the 1.5micron wavelength with the following significant differences. First, theabsorption cross section of the Si nanocrystals is larger than that ofthe Er^(3,) ions by more than 3 orders of magnitude. Second, asexcitation occurs via Auger-type interaction between carriers in the Sinanocrystals and Er³⁺ ions, incident photons need not be in resonancewith one of the narrow absorption bands of Er³⁺. However, existingapproaches to developing such Si nanocrystals have only been successfulat producing concentrations of up to 0.3 atomic percent of the rareearth element, which is not sufficient for practical applications.

In general, manufacture of type IV semiconductor nanocrystals doped witha rare earth element is done by ion implantation of silicon ions into asilicon oxide layer, followed by high temperature annealing to grow thesilicon nanocrystals and to reduce the ion implantation damage. Theimplantation of Si ions is followed by an ion implantation of the rareearth ions into the annealed silicon nanocrystal oxide layer. Theresulting layer is again annealed to reduce the ion implant damage andto optically activate the rare-earth ion.

There are several problems with this method: i) it results in adecreased layer surface uniformity due to the ion implantation; ii) itrequires an expensive ion implantation step; iii) it fails to achieve auniform distribution of group IV semiconductor nanocrystals andrare-earth ions unless many implantation steps are carried out; and iv)it requires a balance between reducing the ion implant damage by thermalannealing while trying to maximise the optically active rare-earth.

To diminish the above drawbacks, Plasma Enhanced Chemical VaporDeposition (PECVD) has been utilised to make type IV semiconductornanocrystal layers. The prepared layers are then subjected to arare-earth ion implantation step and a subsequent annealing cycle toform the IV semiconductor nanocrystals, and to optically activate therare-earth ions that are doped in the nanocrystal region. Unfortunately,the layers prepared with this method are still subjected to animplantation step, which results in a decrease in surface uniformity.

Another PECVD method that has been used to obtain a doped type IVsemiconductor crystal layer consists of co-sputtering together both thegroup IV semiconductor and rare-earth metal. In this method, the groupIV semiconductor and a rare-earth metal are placed into a vacuum chamberand exposed to an Argon ion beam. The argon ion beam sputters off thegroup IV semiconductor and the rare-earth metal, both of which aredeposited onto a silicon wafer. The film formed on the silicon wafer isthen annealed to grow the nanocrystals and to optically activate therare-earth ions. As the rare earth metal is in solid form, the argon ionbeam (plasma) is only able to slowly erode the rare earth, which leadsto a low concentration of rare earth metal in the deposited film. Whilehigher plasma intensity could be used to more quickly erode the rareearth metal and increase the rare earth concentration in the film, ahigher intensity plasma damages the film or the group IV semiconductorbefore it is deposited. The plasma intensity is therefore kept low topreserve the integrity of the film, therefore limiting the rare earthconcentration in the film. The doped group IV semiconductor nanocrystallayers made through this method have the drawbacks that: i) the layerdoes not have a very uniform distribution of nanocrystals and rare-earthions, ii) the layer suffers from upconversion efficiency losses due torare-earth clustering in the film, and iii) the concentration of rareearth metal in the layer is limited by the plasma intensity, which iskept low to avoid damaging the layer.

The concentration of the rare earth element in semiconductor nanocrystallayers is preferably as high as possible, as the level ofphotoelectronic qualities of the film, such as photoluminescence, isproportional to the concentration. One problem encountered when a highconcentration of rare earth element is present within the semiconductorlayer is that when two rare earth metals come into close proximity withone another, a quenching relaxation interaction occurs that reduces thelevel of photoelectronic dopant response observed. The concentration ofrare earth element within a semiconductor film is thus balanced to be ashigh as possible to offer the most fluorescence, but low enough to limitthe quenching interactions.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a doped semiconductornanocrystal layer, the doped semiconductor nanocrystal layer comprising(a) a group IV oxide layer which is free of ion implantation damage, (b)semiconductor nanocrystals distributed in the group IV semiconductoroxide layer, and (c) from 0.5 to 15 atomic percent of one or more rareearth elements. The one or more rare earth element are: (i) dispersed onthe surface of the semiconductor nanocrystal and (ii) distributedsubstantially equally through the thickness of the group IV oxide layer.

In another aspect, the present invention provides a semiconductorstructure comprising a substrate, on which substrate is deposited one ormore of the doped semiconductor nanocrystal layer described above.

In another aspect, the present invention provides a process forpreparing a doped semiconductor nanocrystal layer, the processcomprising:

(a) introducing (i) a gaseous mixture of a group IV element precursorand molecular oxygen, and (ii) a gaseous rare earth element precursor,in a plasma stream of a Plasma Enhanced chemical Vapor Deposition(PECVD) instrument to form a semiconductor rich group IV oxide layerdoped with a rare earth element, and

(b) annealing the semiconductor rich group IV oxide layer doped with arare earth element at a temperature of from 600° C. to 1000° C.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying figures which illustrate preferredembodiments of the present invention by way of example.

DESCRIPTION OF THE FIGURES

Embodiments of the invention will be discussed with reference to thefollowing Figures:

FIG. 1 is a diagram of a semiconductor structure comprising a substrate,a doped semiconductor nanocrystal layer, and a current injection layer;

FIG. 2 is a diagram of a superlattice semiconductor structure comprisinga substrate and alternating doped semiconductor nanocrystal layers anddielectric layers; and

FIG. 3 is a diagram of a Pulse Laser Deposition apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Doped Semiconductor Nanocrystal Layer

The doped semiconductor nanocrystal layer of the invention comprises agroup IV oxide layer in which is distributed semiconductor nanocrystals.The group IV element used to prepare the layer is preferably selectedfrom silicon, germanium, tin and lead, and the group IV semiconductoroxide layer is more preferably silicon dioxide. The group IV oxide layerpreferably has a thickness of from 1 to 2000 nm, for example of from 80to 2000 nm, from 100 to 250 nm, from 30 to 50 nm, or from 1 to 10 nm.

The semiconductor nanocrystals that are dispersed within the group IVsemiconductor oxide layer are preferably the nanocrystals of a group IVsemiconductor, e.g. Si or Ge, or a group II-VI semiconductor, e.g. ZnO,ZnS, ZnSe, CaS, CaTe or CaSe, or of a group III-V semiconductor, e.g.GaN, GaP or GaAs. The nanocrystals are preferably from 1 to 10 nm insize, more preferably from 1 to 3 nm in size, and most preferably from 1to 2 nm in size. Preferably, the semiconductor material is presentwithin the group IV semiconductor oxide layer in a concentration of from30 to 50 atomic percent, more preferably in a total concentration of 37to 47 atomic percent, and most preferably in a concentration of from 40to 45 atomic percent.

The one or more rare earth element that is dispersed on the surface ofthe semiconductor nanocrystal can be selected to be a lanthanideelement, such as cerium, praseodymium, neodymium, promethium,gadolinium, erbium, thulium, ytterbium, samarium, dysprosium, terbium,europium, holmium, or lutetium, or it can be selected to be an actinideelement, such as thorium. Preferably, the rare earth element is selectedfrom erbium, thulium, and europium. The rare earth element can, forexample, take the form of an oxide or of a halogenide. Of thehalogenides, rare earth fluorides are preferred as they display moreintense fluorescence due to field distortions in the rare earth-fluoridematrix caused by the high electronegativity of fluorine atoms. Mostpreferably, the rare earth element is selected from erbium oxide, erbiumfluoride, thulium oxide, thulium fluoride, europium oxide and europiumfluoride.

The one or more rare earth element is preferably present in the group IVsemiconductor oxide layer in a concentration of 0.5 to 15 atomicpercent, more preferably in a concentration of 5 to 15 atomic percentand most preferably in a concentration of 10 to 15 atomic percent. Whilesuch a high concentration of rare earth element has led to importantlevels of quenching reactions in previous doped semiconductor materials,the doped semiconductor nanocrystal layer of the present invention canaccommodate this high concentration as the rare earth element isdispersed on the surface of the semiconductor nanocrystal, whichnanocrystal offers a large surface area. The reduced amount of quenchingreactions between the rare earth element and the proximity of the rareearth element to the semiconductor nanocrystal provide the basis for adoped semiconductor nanocrystal layer that offers improvedoptoelectronic properties.

Semiconductor Structure

Using the doped semiconductor nanocrystal layer described above, amultitude of semiconductor structures can be prepared. For example, asemiconductor structure is shown in FIG. 1, in which one or more layers33 of the doped semiconductor nanocrystal layer are deposited on asubstrate 31.

The substrate on which the semiconductor nanocrystal layer is formed isselected so that it is capable of withstanding temperatures of up to1000° C. Examples of suitable substrates include silicon wafers or polysilicon layers, either of which can be n-doped or p-doped (for examplewith 1×10²⁰ to 5×10²¹ of dopants per cm³), fused silica, zinc oxidelayers, quartz and sapphire substrates. Some of the above substrates canoptionally have a thermally grown oxide layer, which oxide layer can beof up to about 2000 nm in thickness, a thickness of 1 to 20 nm beingpreferred. The thickness of the substrate is not critical, as long asthermal and mechanical stability is retained.

The semiconductor structure can comprise a single or multiple dopedsemiconductor nanocrystal layers, each layer having an independentlyselected composition and thickness. By using layers having differentrare earth elements, a multi-color emitting structure can be prepared.For example, combining erbium, thulium and europium in a singlesemiconductor structure provides a structure that can fluoresce at thecolors green (erbium), blue (thulium), and red (europium).

When two or more doped semiconductor nanocrystal layers are used in asingle semiconductor structure, the layers can optionally be separatedby a dielectric layer. Examples of suitable dielectric layers includesilicon dioxide, silicon nitrite and silicon oxy nitrite. The silicondioxide dielectric layer can also optionally comprise semiconductornanocrystals. The dielectric layer preferably has a thickness of from 1to 10 nm, more preferably of 1 to 3 nm and most preferably of about 1.5nm. The dielectric layer provides an efficient tunnelling barrier, whichis important for obtaining high luminosity from the semiconductorstructure.

The semiconductor structure can also have an Indium Tin Oxide (ITO)current injection layer (34) overtop the one or more doped semiconductornanocrystal layers. The ITO layer preferably has a thickness of from 150to 300 nm. Preferably, the chemical composition and the thickness of theITO layer is such that the semiconductor structure has a conductance offrom 30 to 70 ohms cm.

The thickness of the semiconductor structure is preferably 2000 nm orless, and the thickness will depend on the thickness of the substrate,the number and thickness of the doped semiconductor nanocrystal layerspresent, the number and the thickness of the optional dielectric layers,and the thickness of the optional ITO layer.

One type of preferred semiconductor structure provided by an embodimentof the present invention is a superlattice structure, shown by way ofexample in FIG. 2, which structure comprises multiple layers ofhetero-material 20 on a substrate 11. Multiple doped semiconductornanocrystals layers having a thickness of from 1 nm to 10 nm aredeposited on the substrate 12 and 14, and the doped semiconductornanocrystals layers can comprise the same or different rare earthelements. Optionally, the doped semiconductor nanocrystal layers areseparated by dielectric layers 13 of about 1.5 nm in thickness, and anITO current injection layer (not shown) can be deposited on top of themultiple layers of the superlattice structure. There is no maximumthickness for the superlattice structure, although a thickness of from250 to 2000 nm is preferred and a thickness of from 250 to 750 nm ismore preferred.

Preparation of the Doped Semiconductor Nanocrystal Layer

The preparation of the doped semiconductor nanocrystal layer comprisesthe following two general steps:

(a) the simultaneous deposition of a semiconductor rich group IV oxidelayer and of one or more rare earth element; and

(b) the annealing of the semiconductor rich group IV oxide layerprepared in (a) to form semiconductor nanocrystals.

The semiconductor rich group IV oxide layer comprises a group IV oxidelayer, which group IV oxide is preferably selected from SiO₂ orGeO_(2i), in which group IV oxide layer is dispersed a rare earthelement and a semiconductor, which semiconductor can be the same as, ordifferent than, the semiconductor that forms the group IV oxide layer.

By “semiconductor rich”, it is meant that an excess of semiconductor ispresent, which excess will coalesce to form nanocrystals when thesemiconductor rich group IV oxide layer is annealed. Since the rareearth element is dispersed within the oxide layer when the nanocrystalsare formed, the rare earth element becomes dispersed on the surface ofthe semiconductor nanocrystals upon nanocrystal formation.

Since the semiconductor rich group IV oxide layer and the one or morerare earth element are deposited simultaneously, ion implantation of therare earth element is avoided. As such, the group IV oxide layer surfaceis free of the damage associated with an implantation process. Also,since the rare earth element is deposited at the same time as thesemiconductor rich group IV oxide layer, the distribution of the rareearth element is substantially constant through the thickness of thegroup IV oxide layer.

The deposition of the semiconductor rich group IV oxide layer doped withone or more rare earth elements is preferably carried out byPlasma-Enhanced Chemical Vapor Deposition (PECVD) or by Pulse LaserDeposition (PLD). The above two methods each have their respectiveadvantages for preparing the semiconductor rich group IV oxide layerdoped with one or more rare earth elements, and the methods aredescribed below.

Pulse Laser Deposition

Pulse laser deposition is advantageous for the deposition of thesemiconductor rich group IV oxide layer doped with one or more rareearth elements as it permits the deposition of a wide variety ofsemiconductors and a wide variety of rare earth elements.

Referring now to FIG. 3, which shows by way of a diagram a typical setup of a pulse laser deposition apparatus, the pulse laser depositionapparatus consists of a large chamber 41, which can be evacuated down toat least 10⁻⁷ bars or pressurized with up to 1 atmosphere of a gas suchas oxygen, nitrogen, helium, argon, hydrogen or combinations thereof.The chamber has at least one optical port 42 in which a pulse laser beam45 can be injected to the chamber and focused down onto a suitabletarget 44. The target is usually placed on a carrousel 43 that allowsthe placement of different target samples into the path of the pulselaser focus beam. The carrousel is controlled so that multiple layers ofmaterial can be deposited by the pulse laser ablation of the target. Theflux of the focused pulse laser beam is adjusted so that the targetablates approximately 0.1 nm of thickness of material on a substrate 47,which can be held perpendicular to the target and at a distance of 20 to75 millimetres above the target. This flux for instance is in the rangeof 0.1 to 20 joules per square cm for 248 nm KrF excimer laser and has apulse width of 20-45 nanosecond duration. The target can be placed on ascanning platform so that each laser pulse hits a new area on thetarget, thus giving a fresh surface for the ablation process. This helpsprevent the generation of large particles, which could be ejected in theablation plume 46 and deposited on to the substrate. The substrate isusually held on a substrate holder 48, which can be heated from roomtemperature up to 1000° C. and rotated from 0.1 to 30 RPM depending onthe pulse rate of the pulse laser, which in most cases is pulsed between1- 10 Hz. This rotation of the substrate provides a method of generatinga uniform film during the deposition process. The laser is pulsed untilthe desired film thickness is met, which can either be monitored in realtime with an optical thickness monitor or quartz crystal microbalance ordetermined from a calibration run in which the thickness is measuredfrom a given flux and number of pulses. Pulse laser deposition can beused for depositing layers of from 1 to 2000 nm in thickness.

For the preparation of a semiconductor rich group IV oxide layer dopedwith one or more rare earth elements, the target that is ablated iscomposed of mixture of a powdered group IV binding agent, a powderedsemiconductor that will form the nanocrystal, and a powdered rare earthelement. The ratio of the various components found in the dopedsemiconductor nanocrystal layer is decided at this stage by controllingthe ratio of the components that form the target. Preferably, themixture is placed in a hydraulic press and pressed into a disk of 25 mmdiameter and 5 mm thickness with a press pressure of at least 500 Psiwhile being heated to 700° C. The temperature and pressure can beapplied, for example, for one hour under reduced pressure (e.g. 10⁻³bars) for about one hour. The press pressure is then reduced and theresulting target is allowed to cool to room temperature.

The group IV binding agent can be selected to be a group IV oxide (e.g.silicon oxide, germanium oxide, tin oxide or lead oxide), oralternatively, it can be selected to be a group IV element (e.g.silicon, germanium, tin or lead). When the group IV binding agent is agroup IV oxide, the binding agent, the semiconductor and the rare earthelement are combined to form the target, and the pulse laser depositionis carried out in the presence of any one of the gases listed above. Ifa group IV element is used as the group IV binding agent instead, thepulse laser deposition is carried out under an oxygen atmosphere,preferably at a pressure of from 1×10⁻⁴ to 5×10⁻³ bar, to transform someor all of the group IV element into a group IV oxide during the laserdeposition process. When the semiconductor element which is to form thenanocrystals is selected to be a group II-VI semiconductor (e.g. ZnO,ZnS, ZnSe, CaS, CaTe or CaSe) or a group III-V semiconductor (e.g. GaN,GaP or GaAs), the oxygen concentration is kept high to insure that allof the group IV element is fully oxidized. Alternatively, if thenanocrystals to be formed comprise the same group IV semiconductorelement that is being used as the binding agent, the oxygen pressure isselected so that only part of the group IV element is oxidized. Theremaining non-oxidized group IV element can then coalesce to formnanocrystals when the prepared semiconductor rich group IV oxide layeris annealed.

The powdered rare earth element that is used to form the target ispreferably in the form of a rare earth oxide or of a rare earthhalogenide. As mentioned above, the rare earth fluoride is the mostpreferred of the rare earth halogenides.

Pulse laser deposition is useful for the subsequent deposition of two ormore different layers. Multiple targets can be placed on the carrouseland the pulse laser can be focussed on different targets during thedeposition. Using this technique, layers comprising different rare earthelements can be deposited one on top of the other to preparesemiconductor structures as described earlier. Different targets canalso be used to deposit a dielectric layer between the semiconductorrich group IV oxide layers, or to deposit a current injection layer ontop of the deposited layers. Pulse laser deposition is the preferredmethod for preparing the superlattice semiconductor structure describedabove.

Preparation of the semiconductor rich group IV oxide layer doped withone or more rare earth elements can of course be carried out withdifferent pulse laser deposition systems that are known in the art, theabove apparatus and process descriptions being provided by way ofexample.

Plasma Enhanced Chemical Vapor Deposition

PECVD is advantageous for the deposition of the semiconductor rich groupIV oxide layer doped with one or more rare earth element, as it permitsthe rapid deposition of the layer. The thickness of the semiconductorrich group IV oxide layer doped with one or more rare earth elementprepared with PECVD is 10 nm or greater, more preferably from 10 to 2000nm.

Formation of anon-doped type IV semiconductor nanocrystal layer throughchemical vapor deposition has been described, for example, by J. Sin, M.Kim, S. Seo, and C. Lee [Applied Physics Letters, (1998), Volume 72, 9,1092-1094], the disclosure of which is hereby incorporated by reference.

In this embodiment, the doped semiconductor nanocrystal layer isprepared by incorporating a rare-earth precursor into the PECVD streamabove the receiving heated substrate on which the semiconductor film isgrown. PECVD can be used to prepare the doped semiconductor nanocrystallayer where the semiconductor nanocrystal is a silicon or a germaniumnanocrystal, and where the rare earth element is a rare earth oxide.

In the PECVD process, a group IV element precursor is mixed with oxygento obtain a gaseous mixture where there is an atomic excess of the groupIV element. An atomic excess is achieved when the ratio of oxygen togroup IV element is such that when a group IV dioxide compound isformed, there remains an excess amount of the group IV element. Thegaseous mixture is introduced within the plasma stream of the PECVDinstrument, and the silicon and the oxygen are deposited on a substrateas a group IV dioxide layer in which a group IV atomic excess is found.It is this excess amount of the group IV element that coalesces duringthe annealing step to form the group IV nanocrystal. For example, toprepare a silicon dioxide layer in which silicon nanocrystals isdispersed, a silicon rich silicon oxide (SRSO) layer is deposited on thesubstrate.

The group IV element precursor can contain, for example, silicon,germanium, tin or lead, of which silicon and germanium are preferred.The precursor itself is preferably a hydride of the above elements. Aparticularly preferred group IV element precursor is silane (SiH₄).

The ratio (Q) of group IV element precursor to oxygen can be selected tobe from 3:1 to 1:2. If an excess of group IV element precursor hydrideis used, the deposited layer can contain hydrogen, for example up toapproximately 10 atomic percent hydrogen. The ratio of the flow rates ofthe group IV element precursor and of oxygen can be kept, for example,between 2:1 and 1:2.

Also introduced to the plasma stream is a rare earth element precursor,which precursor is also in the gaseous phase. The rare earth precursoris added to the plasma stream at the same time as the group IV elementprecursor, such that the rare earth element and the group IV element aredeposited onto the substrate simultaneously. Introduction of the rareearth precursor as a gaseous mixture provides better dispersion of therare earth element within the group IV layer.

Preferably, presence of oxygen in the plasma stream and in the depositedlayer leads to the deposition of the rare earth element in the form of arare earth oxide.

The rare earth element precursor comprises one or more ligands. Theligand can be neutral, monovalent, divalent or trivalent. Preferably,the ligand is selected so that when it is coordinated with the rareearth element, it provides a compound that is volatile, i.e. that entersthe gaseous phase at a fairly low temperature, and without changing thechemical nature of the compound. The ligand also preferably comprisesorganic components that, upon exposure to the plasma in the PECVDapparatus, will form gaseous by-products that can be removed through gasflow or by reducing the pressure within the PECVD apparatus. When theorganic components of the ligand are conducive to producing volatileby-products (e.g. CO₂, O₂) less organic molecules are incorporated intothe deposited layer. Introduction of organic molecules into thedeposited layer is generally not beneficial, and the presence of organicmolecules is sometimes referred to as semiconductor poisoning.

Suitable ligands for the rare earth element can include acetatefunctions, for example 2,2,6,6-tetramethyl-3,5 heptanedione,acetylacetonate, flurolacetonate,6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione,I-propylcyclopentadienyl, cyclopentadienyl, and n-butylcyclopentadienyl.Preferred rare earth metal precursor includetris(2,2,6,6-tetramethyl-3,5-heptanedionato) erbium(III), erbium (III)acetylacetonate hydrate, erbium (III) flurolacetonate,tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate)erbium(III), tris(i-propylcyclopentadienyl)erbium (III),Tris(cyclopentadienyl)erbium (III), andtris(n-butylcyclopentadienyl)erbium (III). A particularly preferred rareearth element precursor is tris(2,2,6,6-tetramethyl-3,5-heptanedionato)erbium(III) (Er⁺³ [(CH₃)₃CCOCH=COC(CH₃)₃]₃), which is also referred toas Er+³ (THMD)₃.

If the rare earth element precursor is not in the gaseous phase at roomtemperature, it must be transferred to the gaseous phase, for example,by heating in an oven kept between 80° C. and 110° C. The gaseous rareearth element precursor is then transferred to the plasma stream with aninert carrier gas, such as argon. The gaseous rare earth elementprecursor is preferably introduced to the plasma at a position that isbelow a position where the group IV element containing compound isintroduced to the plasma. Use can be made of a dispersion mechanism, forexample a dispersion ring, to assist in the dispersion of the gaseousrare earth element precursor in the plasma.

In order to obtain a more even deposition of the doped type IV oxidelayer, the substrate can be placed on a sceptre that rotates duringdeposition. A circular rotation of about 3 rpm is suitable forincreasing the uniformity of the layer being deposited.

An Electron Cyclotron Resonated (ECR) reactor is suitable for producingthe plasma used in the PECVD method described above. ECR is a particularmethod of generating plasma, where the electrons have a spiral motioncaused by a magnetic field, which allows a high density of ions in alow-pressure region. The high ion density with low pressure isbeneficial for deposition, as the rare earth metal precursor can bestripped of its organic components and incorporated uniformly and in ahigh concentration. The plasma used in the PECVD method can comprise,for example, argon, helium, neon or xenon, of which argon is preferred.

The PECVD method is carried out under a reduced pressure, for example1×10⁻⁷ torr, and the deposition temperature, microwave power and scepterbias can be kept constant. Suitable temperature, microwave and scepterbias values can be selected to be, for example, 300° C., 400 W and−200V_(DC), respectively.

The semiconductor rich group IV oxide layer doped with one or more rareearth element can be grown at different rates, depending on theparameters used. A suitable growth rate can be selected to be about 60nm per minute, and the semiconductor rich group IV oxide layer can havea thickness of from 10 to 2000 nm, more preferably of from 100 to 250nm.

Preparation of the semiconductor rich group IV oxide layer doped withone or more rare earth elements can of course be carried out withdifferent plasma enhanced chemical vapor deposition systems that areknown in the art, the above apparatus and process descriptions beingprovided by way of example.

Annealing Step

After the semiconductor rich group IV oxide layer doped with one or morerare earth element has been prepared, the doped type IV oxide layer isannealed, optionally under flowing nitrogen (N₂), in a Rapid ThermalAnneal (RTA) furnace, at from about 600° C. to about 1000° C., morepreferably from 800° C. to 950° C., from 5 minutes to 30 minutes, morepreferably from 5 to 6 minutes. It is during the annealing step that theatomic excess of semiconductor is converted into semiconductornanocrystals.

When PECVD is used to prepare the semiconductor rich group IV oxidelayer doped with one or more rare earth element, the annealing step canalso be carried out under an oxygen atmosphere to insure oxidation ofthe rare earth element, or under a reduced pressure in order tofacilitate the removal of any volatile by-products that might beproduced.

The amount of excess semiconductor in the group IV oxide layer and theanneal temperature dictate the size and the density of the semiconductornanocrystal present in the final doped semiconductor nanocrystal layer.

Since the rare earth element is well dispersed through the depositedgroup IV semiconductor oxide layer, when the nanocrystals are formedduring the annealing step, the rare earth element becomes localised onthe surface of the nanocrystals. Since the nanocrystals provide a largesurface area on which the rare earth element can be dispersed, theconcentration of the rare earth element can be quite elevated, whileretaining good photoelectronic properties.

The following examples are offered by way of illustration and not by wayof limitation.

EXAMPLES Example 1

Silane (SiH₄) and Oxygen (O₂) are added to an argon plasma streamproduced by an Electron Cyclotron Resonated (ECR) reactor via dispersionring. The ratio (Q) of silane to oxygen has been varied between3:1,1.7:1,1.2:1,1:1.9, and 1:2. An erbium precursor(Tris(2,2,6,6-tetramethyl-3,5-heptanedionato) erbium(III) [Er⁺³(THMD)₃])is placed in a stainless steel oven held between 90 and 110° C.

A carrier gas of Ar is used to transport the Er precursor from the oventhrough a precision controlled mass-flow controller to a dispersion ringbelow the Silane injector and above the heated substrate. The instrumentpressure is kept at about 1×10⁻⁷ torr. The substrates used are eitherfuse silica or silicon wafers on which is thermally grown an oxide layerof 2000 nm thickness. The deposition temperature, the microwave powerand the sceptre bias are kept constant at 300° C., 400 W and −200V_(DC).The SiH₄, Ar flow rates were adjusted while keeping the O₂ flow rate at20 militorr sec⁻¹ for the various excess silicon content. The Er/Ar flowrate was adjusted to the vapor pressure generated by the temperaturecontrolled oven for the desired erbium concentration. The film is grownat a rate of 60 nm per minute and thickness has been grown from 250 nmto 2000 nm thick. The scepter was rotated at 3 rpm during the growth tohelp in uniformity of film. After deposition, the samples are annealedat 950° C. -1000° C. for 5-6 minutes under flowing nitrogen (N₂) in aRapid Thermal Anneal (RTA) furnace.

Example 2

An ablation target is fabricated by combining powdered silicon, powderedsilicon dioxide and powdered erbium oxide, the prepared powder mixturecomprising 45% silicon, 35% silicon oxide and 20% erbium oxide. Eachpowder component has a size of about 300 mesh. The mixture is placedinto a ball mill and ground for approximately 5 to 10 minutes. Themixture is then placed into a 25 mm diameter by 7 mm thick mould, placedinto a hydraulic press, and compressed for 15 minutes at 500 psi. Theobtained target is then placed into an annealing furnace and heated to1200° C. in a forming gas atmosphere of 5% H₂ and 95% N₂ for 30 minutes.The Target is cooled down to room temperature and then reground in aball mill for ten minutes. The mixture is then again placed in a mould,compressed and annealed as described above. The obtained target isplaced onto a target holder inside a vacuum chamber. A silicon substrate[n-type, <110> single crystal, 0.1-0.05 Ωcm conductivity] of 50 mmdiameter and 0.4 cm thickness is placed on a substrate holder parallelto and at a distance of 5.0 cm above the surface of the target. Thesubstrate is placed onto a substrate support that is heated at 500° C.,and the substrate is rotated at a rate of 3 rpm during the deposition.The vacuum chamber is evacuated to a base pressure of 1×10⁻⁷ torr andthen back filled with 20×10⁻³ torr of Ar. An excimer laser (KrF 248 nm)is focused on to the target at an energy density of about 10 JCm⁻² andat a glancing angle of 40° to the vertical axis, such that a 0.1 nm filmis generated per pulse. The target is rotated at 5 rpm, duringdeposition in order to have a fresh target surface for each ablationpulse. After a 100 nm layer is deposited on the substrate, the newlydeposited film is annealed at temperature of from 900° C. to 950° C. for5 minutes to form silicon nanocrystals in the Silicon Rich Silicon Oxide(SRSO).

The substrate is reintroduced in the vacuum chamber, 20 and the targetis replaced with an Indium Tin Oxide (ITO) target. The atmosphere insidethe vacuum chamber is set to 2×10⁻³torr of O₂, and the substrate isheated to 500° C. and rotated at 3 rpm. A 100 nm ITO layer is depositedon top of the annealed rare earth doped SRSO film.

All publications, patents and patent applications cited in thisspecification are herein incorporated by reference as if each individualpublication, patent or patent application were specifically andindividually indicated to be incorporated by reference. The citation ofany publication is for its disclosure prior to the filing date andshould not be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it is readily apparent to those of ordinary skill in theart in light of the teachings of this invention that certain changes andmodifications may be made thereto without departing from the spirit orscope of the appended claims.

It must be noted that as used in this specification and the appendedclaims, the singular forms “a”, “an”, and “the” include plural referenceunless the context clearly dictates otherwise. Unless defined otherwiseall technical and scientific terms used herein have the same meaning ascommonly understood to one of ordinary skill in the art to which thisinvention belongs.

1-45. (canceled)
 46. A process for preparing a doped semiconductornanocrystal layer, the process comprising: (a) introducing (i) a gaseousmixture of a group IV element precursor and molecular oxygen, and (ii) agaseous rare earth element precursor, at the same time in a plasmastream of a Plasma Enhanced chemical Vapor Deposition (PECVD)instrument, whereby the rare earth element and the group IV element aredeposited onto a substrate simultaneously to form a semiconductor richgroup IV oxide layer doped with a rare earth element, and (b) annealingthe semiconductor rich group IV oxide layer doped with a rare earthelement at a temperature of from 600° C. to 1000° C., whereby atomicexcess of the group IV element is converted into semiconductornanocrystals; and whereby the rare earth elements are dispersed throughthe semiconductor rich group IV oxide layer when the semiconductornanocrystals are formed, and whereby the rare earth elements arelocalized on the surface of the nanocrystals.
 47. A process according toclaim 46, wherein the group IV element precursor is a hydride of a groupIV element.
 48. A process according to claim 46, wherein the group IVelement precursor comprises silicon, germanium, tin or lead.
 49. Aprocess according to claim 46, wherein the group IV element precursor issilane.
 50. A process according to claim 46, wherein the ratio of thegroup IV element precursor and of the molecular oxygen is selected toobtain the semiconductor rich group IV oxide layer with 30 to 50 atomicpercent of excess semiconductor.
 51. A process according to claim 46,wherein the rare earth element precursor comprises a rare earth elementselected from cerium, praseodymium, neodymium, promethium, gadolinium,erbium, thulium, ytterbium, samarium, dysprosium, terbium, europium,holmium, lutetium, and thorium.
 52. A process according to claim 46,wherein the rare earth element precursor comprises erbium, thulium oreuropium.
 53. A process according to claim 46, wherein the rare earthelement precursor comprises a ligand selected from2,2,6,6-tetramethyl-3,5-heptan-edione, acetylacetonate, flurolacetonate,6,6,7,7,8,8,8-heptafluoro-2,2-di-methyl-3,5-octanedione,i-propylcyclopentadienyl, cyclopentadienyl, and n-butylcyclopentadienyl;whereby the rare earth element precursor with the ligand forms acompound, which is volatile and enters the gaseous phase at a relativelylow temperature without changing the chemical nature of the compound,and comprises organic components that, upon exposure to the plasma inthe PECVD apparatus, will form gaseous by-products that can be removedthrough gas flow or by reducing the pressure within the PECVD apparatus.54. A process according to claim 46, wherein the rare earth elementprecursor is selected from tris(2,2,6,6-tetramethyl-3,5-heptanedionato)erbium(III), erbium (III) acetylacetonate hydrate, erbium (III)flurolacetonate,tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedi-onate)erbium(III), tris(i-propylcyclopentadienyl)erbium (III),Tris(cyclopentadienyl)erbium (III), andtris(n-butylcyclopentadienyl)erbi-um (III).
 55. A process according toclaim 46, wherein the semiconductor rich group IV oxide layer isannealed at a temperature of from 800 to 950° C.
 56. (canceled)
 57. Theprocess according to claim 46, wherein step (b) is carried out under anoxygen atmosphere to insure oxidation of the rare earth element, orunder a reduced pressure in order to facilitate the removal of anyvolatile by-products that are produced.
 58. The process according toclaim 46, further comprising heating the rare earth element precursor toensure it is in a gaseous state.
 59. The process according to claim 58,wherein the rare earth element precursor is heated in an oven at between80° C. and 110° C.
 60. The process according to claim 46, wherein thegaseous rare earth element precursor is introduced to the plasma streamwith an inert carrier gas.
 61. The process according to claim 46,wherein the gaseous rare earth element precursor is introduced to theplasma at a position that is below a position where the group IV elementcontaining compound is introduced to the plasma.
 62. The processaccording to claim 46, wherein step a) includes use of a dispersion ringto assist in the dispersion of the gaseous rare earth element precursorin the plasma.
 63. The process according to claim 46, wherein step a)includes placing the substrate on a rotating scepter to obtain a moreeven deposition of the semiconductor rich group IV oxide layer.
 64. Theprocess according to claim 46, further comprising producing the plasmain an Electron Cyclotron Resonated (ECR) reactor, wherein electrons havea spiral motion caused by a magnetic field, which allows a high densityof ions in a low-pressure region, whereby a rare earth metal componentof the rare earth element precursor is stripped of organic components ofthe rare earth element precursor and incorporated uniformly and in ahigh concentration.